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New Instrument Package to Expand Space Telescope's Vision

Photonics.com
Feb 2002
BALTIMORE, Feb. 21 -- NASA's fourth servicing mission for the Hubble Space Telescope, scheduled to lift off on Space Shuttle Columbia Feb. 28, will give the orbital observatory a series of midlife upgrades that includes the advanced camera for surveys (ACS), a new instrument package that will increase Hubble's already formidable capacity for discoveries tenfold, according to the leader of the team that built it.

"If you had two fireflies six feet apart in Tokyo, Hubble's vision with ACS will be so fine that it will be able to tell from Washington, D.C., that they were two different fireflies instead of one," says Holland Ford, professor of astronomy in the Krieger School of Arts and Sciences at The Johns Hopkins University and leader of the team that built the ACS over a five-year-period.
Ford thinks there's an outside chance that the ACS might even be powerful enough to obtain "direct evidence" -- i.e., an image of some type -- of planets in other, nearby solar systems. Although planets have been detected around many stars, all of them have been inferred through the gravitational wobbles they impart to their stars, rather than detected through a direct image of the planets themselves.

The ACS will replace an instrument currently in Hubble known as the faint object camera, which is the last of Hubble's original instruments. After catching Hubble with the shuttle's robot arm and securing it in the shuttle's payload bay, spacewalking astronauts will open the servicing doors on Hubble, remove the faint object camera, and install the ACS.

The ACS weighs 870 pounds and is "about the size of an old-fashioned phone booth," according to Ford. Inside the ACS are three electronic cameras (the wide-field, high-resolution, and solar blind cameras), and a range of filters, polarizers, dispersers and other astronomical tools. ACS can detect radiation ranging from the ultraviolet portion of the spectrum, through visible light, to the near-infrared.


These simulated images show a distant, massive cluster of galaxies as seen by Hubble's current imaging camera, the Wide Field Planetary Camera 2 (WFPC2), and by the ACS's wide field camera. The cluster is "observed" by the two instruments for the same length of time and through the same color filters. The striking differences in appearance reflect differing camera sensitivities, resolution and fields of view. The ACS image reveals a wealth of detail not seen with WFPC2, including several faint arc-like features that result from the massive cluster gravitationally "lensing" the light of much more distant background galaxies. The combination of ACS's improved sensitivity and larger field of view is expected to produce a tenfold improvement in discovery efficiency for Hubble.


In comparison to the Wide Field Camera II, another instrument already in use in Hubble, the ACS will provide two times the observational area, two times the resolution, and four times the sensitivity. For example, astronomers use Hubble to probe the distant reaches of the universe in a project known as a deep-sky survey. If they probe to the same distances as previous surveys, researchers should be able to finish their work approximately ten times faster, reducing their observation time on the telescope from 20 days to just a few days.

ACS also contains an instrument known as a coronagraph that will allow astronomers to block out small bright sources of light in order examine the details of structures around the light sources. Ford noted that this may allow astronomers to search for warps and gaps in the disks of gas and dust surrounding nearby stars that may be early signs of planet formation. The coronagraph will also be very useful to astronomers who study quasars, powerful distant objects in the farthest reaches of the universe that are thought to be highly active black holes in the centers of galaxies.

"We're looking forward to taking images of quasars, and seeing the structures that surround the quasars much better with the ACS's higher resolution and higher sensitivity, but especially with the ACS's ability to block the extremely bright emissions coming from the quasar," explains Ford.

Ford and other astronomers have many other ideas for using the ACS, including taking a closer, more detailed look at the weather on planets in our solar system, and no less ambitious a project than verifying the celestial yardstick astronomers have used for several decades to gauge distances around the universe.

"ACS has a set of filters that lets us take pictures in polarized light, which in effect can allow us to see around corners," says Ford. "We plan to use the polarizers to make some geometric measurements of distances using light echoes from supernovae. This will give us very important checks on how we bootstrap distances across the universe."



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